This invention relates to a process for conditioning an organic pigment using low amounts of polyphosphoric acid at a particular range of temperature, and more particularly to an improved process of finishing organic pigment in a finely-divided form exhibiting high tinctorial strength and tinctorial stability in lacquers and in coloration of plastics.
It is conventional to condition organic pigment such as phthalocyanine, quinacridone, perylene, carbazole violet by a process known as acid swelling. Such a process involves mixing pigment in sulfuric acid of certain ratios. With such ratios of acid, the pigment does not dissolve nor is it crystallized but is caused.to swell and after removal of the liquid and drying, the pigment of a soft textured product having a very fine particle size is obtained. The amount of acid used in this treatment must be sufficient to obtain a stirrable slurry. Generally, a ratio of the order of one part pigment to 3-20 parts sulfuric acid has been found to produce satisfactory results.
U.S. Pat. No. 4,432,796 discloses the use of polyphosphoric acid has to condition pigments. However, the amount of polyphosphoric acid used is 2-20 and preferably 3-8 parts by weight, based on 1 part by weight of the 100% pigment. The concentration of the polyphosphoric acid is 100 to 125%. In all cases, it requires significant amount of acid which increases the production cost in order to provide an efficient agitation and effective transfer from acid swelling reactor to the digestive solvent or water for further treatment.
The present invention provides a method of conditioning an organic crude pigment comprising acid treating under high shear 1 part by weight of the pigment in about 0.5 to 1.9 parts by weight of polyphosphoric acid or polyphosphoric acid methyl ester at a temperature ranging from about 90 to 160xc2x0 C.
Other objects and advantages of the present invention will become apparent from the following description and appended claims.
It has now been found that the conditioning of an organic pigment such as phthalocyanine, quinacridone, perylene, carbazole violet can be achieved in a high shear mixer through acid swelling the crude in about 0.5 to 1.9 fold of concentrated polyphosphoric acid or an acidic polyphosphoric acid methyl ester at 90-160xc2x0 C.
The resulting magma is then digested in water or a water miscible organic solvent to remove the phosphoric acid and provide a finely-divided product exhibiting high tinctorial strength and excellent dispersibility in lacquers and in coloration of plastics.
Suitable high shear mixing under process of the invention includes, for example, using sigma blade kneaders (available from Baker Perkins), MP mixer and twin screw extruder (available from Werner Pfleiderer).
Concentration of polyphosphoric acid and swelling temperature during the attrition are key factors not only in determining the particle size of resulting pigment but also in affecting product shade and cleanness, and must be controlled carefully.
Polyphosphoric acid is added to the crude under high shear and the crude is then charged to the reactor under elevated temperature to obtain a good firm magma. Polyphosphoric acid with concentration between 110 to 125% is used for this process, preferably between 116 to 118%. The concentration of polyphosphoric acid is dependent upon the quantity of diphosphor pentoxide dissolved. Polyphosphoric acid can be purchased commercially in the range of 117 to 120%.
The amount of polyphosphoric acid is approximately 0.5 to 1.9 fold of crude, preferably 1 to 1.5 fold. The acid is mixed with crude to form a molten magma. The amount of acid will determine the physical appearance of the resulting magma which must be wet enough to provide a uniform mixture and prevent dusting from unwetted crude. Higher amounts of polyphosphoric acid do not harm the product quality but will provide a soupy-like magma and increase the cost of production.
The polyphosphoric acid mixture is generally heat to a temperature of from about 90 to 160xc2x0 C., in particular 120-135xc2x0 C. for about 0.5-4 hours. While not wishing to be bound by the theory, we believe that the resulting crude reacts with polyphosphoric acid to form phosphate salt which is often in a state of oily appearance similar to that of flush product. The mixing and shearing by the high shear mixer provide a uniform mass and very often this action does generate internal heat due to friction of particles. The subsequent attrition will then cause intensive rise in reaction temperature and cooling must be applied. In some circumstances, upon cooling the resulting mixture is considerably dry and will crumble and can then be further crushed into powder.
The length of acid swelling or conditioning through,attrition is kept at 1 to 6 hours, preferably at 1 to 3 hours.
The resulting phosphate salt can be removed either by knife at high temperature similar to that of flush or by decanting as normal salt attrition material dependent upon the physical aspect of magma. The reaction medium is then suitably stirred with about 4 to 8 times the quantity of solvent, calculated on the content of pigment, and followed by wet grinding. The deagglomerated crude quinacridone is thereafter heated to a temperature of from 60-150xc2x0 C., preferably 80-120xc2x0 C. for about 3 to 8 hours. The salt magma can be also stirred with about 6 to 10 times of water and followed by wet grinding. The water slurry is filtered, washed with water and the resulting presscake is reslurried into solvent or water and heated to a temperature of from 60-150xc2x0 C., preferably 80-120xc2x0 C. for about 3 to 8 hours.
Suitable wet grinding machines for the process of the invention include, for example, colloid mills, corundum disk mills and similar apparatuses.
Suitable solvents for the process of the invention are preferably lower alcohols, ketones, acids or other basic solvents, for example, methanol, ethanol, n-propabol, isopropanol, n-butanol, iso-butanol, pentanol, hexanol, acetone, methyl ethylketone, methylisobutylketone, glacial acetic acid, dimethylforamide, dimethylsulfoxide, N-methylpyrolidone.
The pigments obtained by the process of the invention are distinguished by high tinctorial strength, a pure shade and a good dispersibility in lacquers and in the coloration of plastics.